You are playing the following Flip Game with your friend: Given a string that contains only these two characters: + and -, you and your friend take turns to flip two consecutive "++" into "--". The game ends when a person can no longer make a move and therefore the other person will be the winner.Write a function to determine if the starting player can guarantee a win.For example, given s = "++++", return true. The starting player can guarantee a win by flipping the middle "++" to become "+--+".Follow up:Derive your algorithm's runtime complexity.

若是有一段"++", 剩下的段和"--"组合 can not win, 那么返回true.

从头到尾试遍了没找到这么一段"++", 返回false.

Time Complexity: exponential.

T(n) = (n-2) * T(n-2) = (n-2) * (n-4) * T(n-4) = O(n!!)

public class Solution {    public boolean canWin(String s) {        for(int i = 1; i